The present invention is directed to the field of firearms and more particularly involves an automatic/semi-automatic hand-held rifle of the type exemplified by the U.S. military M16.
The M-16 automatic rifle and the AR15 semi-automatic rifle have been the standard issue weapons of the U.S. military and civilian police departments for decades. The rifle design was originated by E. M. Stoner and developed by Fairchild Engine and Airplane Company in the 1950's. Modified versions of the M16 designated as the M16A1 and M16A2 are currently in use by armed forces in the U.S. and throughout the world. A civilian semi-automatic version of the M16 designated as the AR15 is sold to civilians by Olympic Arms of Olympia, Wash. When used herein, the phrase "M16" is intended to include all versions of the M16 and AR15 previously and currently being produced.
One of the basic patents on gas-operated firearms was that granted to Browning in 1938, U.S. Pat. No. 2,116,141, which was a divisional application of U.S. Pat. No. 2,093,705. These patents disclose a piston tube assembly for disengaging the barrel from the casing of the gun.
U.S. Pat. No. 2,951,424 issued to E. M. Stoner on Sep. 6, 1960, discloses the M16 bolt and bolt carrier system and the gas operation thereof. This patent discloses a rifle utilizing a gas tube that extends from gas ports in the barrel, back into the receiver of the rifle and into a gas tube pocket or "key" attached to the bolt carrier.
U.S. Pat. No. 3,198,076 to E. M. Stoner, issued Aug. 3, 1965, discloses a gas operated, magazine-fed rifle that can be readily converted to a belt-fed machine gun by inverting the barrel assembly.
U.S. Pat. No. 3,675,534, issued to P. C. Beretta on Jul. 11, 1972, discloses a gas-operated automatic rifle having a piston and stem inside a gas tube with the stem fixedly attached to the bolt carrier.
U.S. Pat. No. 4,358,986, issued to C. Giorgio on Nov. 16, 1982, discloses a gas-operated automatic rifle having a stationary piston and a segmented movable gas cylinder/operating rod assembly including a biasing spring.
U.S. Pat. No. 3,618,457, issued to A. Miller on Nov. 9, 1971, discloses a gas-operated rifle utilizing a gas-operated piston and rod assembly with the piston rod telescopically mounted over a stationary guide rod and being spring-biased.
U.S. Pat. No. 4,765,224, issued Aug. 23, 1988, to M. Morris discloses a modified M16 type of rifle utilizing an extended gas tube receiver on the bolt carrier which maintains telescopic engagement with the gas tube at all times during the firing cycle.
U.S. Pat. No. 4,475,438 to L. Sullivan, issued on Oct. 9, 1984, discloses an open-bolt gas-operated rifle with a short-stroke piston that kicks open the bolt carrier against a biasing spring, using a short-stroke piston movement.
While the aforementioned gas-operated rifles all disclose various means of actuating a bolt in an automatic or semi-automatic rifle, none of these teaches a design for an M16 type of rifle that allows the rifle to be chambered for short pistol-type cartridges such as the 40 S&W cartridge and the 45 caliber ACP cartridge. There is insufficient gas generated in these short "fat" pistol cartridges to fully activate the bolt carriers in these designs. For example, the conventional M16 rifle is normally chambered for the 5.56 NATO rifle cartridge which generates gas pressures in the range of 52,000 CUP. This rifle utilizes a gas port passing through the wall of the barrel which transmits pressurized gases from the firing of the cartridge back to the bolt carrier. The pressurized gases impinge a gas key on the bolt carrier to drive the bolt carrier and bolt backward in the receiver.
The cycling of the bolt carrier assembly in the conventional M16 rifle depends entirely upon the gas pressures generated in the gas-tube assembly, which gases impact the bolt carrier and drive it backward. This gas pressure also communicates through gas vents in the bolt carrier to a piston on the bolt to provide movement of the bolt in the carrier.
The disadvantage of the M16 rifle is its inability to handle relatively low-pressure cartridges such as those used with the short, fat pistol cartridges including the 0.45 ACP, the 0.40 S&W, and the 9 mm. The pressures generated in these short cartridges are in the range of 40,000 CUP as contrasted with the 50,000-55,000 CUP generated with most rifle cartridges. These low pressures in the pistol cartridges are insufficient to cycle the conventional M16 bolt assembly acting through the gas tube/gas key assembly of the M16. One reason these pressures are insufficient to cycle the M16 bolt assembly is because of the large energy requirement in moving the bolt backward in the receiver while simultaneously rotating the bolt out of locking engagement in the chamber, extracting the cartridge, and ejecting it. While simultaneously performing all of these functions, the gas pressure must also move a weighted rod called a buffer and compress a buffer spring telescopically located in the rifle buttstock.
Another disadvantage of the gas-operated M16 currently being manufactured is the fouling of the gas ports in the bolt carrier and the gas rings on the piston end of the bolt. A large amount of residue from the buring powder collects in these small and rather tortured ports and grooves. Also, dirt and moisture from the atmosphere are mixed with the gas from the burning powder in the gas system and residuals are formed in the ports and in the gas rings on the bolt, which eventually clog and jam the weapon. Gas exiting the ports from the bolt also mix with dirt and moisture and cause deposits between the bolt, the chamber, and the receiver, thereby interfering with proper operation of the bolt and bolt carrier in the receiver.
One method of overcoming these difficulties in chambering the M16 rifle for short, low-pressure pistol cartridges is to utilize a blow-back design instead of the gas tube/gas key system used in the conventional M16 rifle. A blow-back system differs from the M16 system in that the bolt is never locked-up in the breech and the bolt cycle is achieved by gas pressure in the chamber directly blowing back the cartridge against the bolt face to cycle the bolt back against the buffer assembly to accomplish spent cartridge ejection and new cartridge loading.